The present invention relates generally to semiconductor integrated circuit devices and, more particularly, to layout schemes of static random access memory (SRAM) cells. The invention also relates to semiconductor memory devices using such cells.
One-port SRAM cells with complementary metal oxide semiconductor (CMOS) configurations are typically designed so that each cell consists essentially of six separate transistors. An exemplary layout of such cells has been disclosed, for example, in JP-A-10-178110 (laid open on Jun. 30, 1998).
In the previously known SRAM cell layout, a semiconductive well region of P type conductivity with inverters formed therein is subdivided into two subregions, which are disposed on the opposite sides of an N-type well region while permitting a well boundary line to extend in a direction parallel to the bit lines.
The quest for higher integration and ultra-fine patterning techniques in modern memory devices requires an optical exposure apparatus or equipment to decrease the wave length of the beams used therein. To this end, the equipment is designed to employ exposure beams of shorter wavelengths, which have advanced from G line to I line, and then further to excimer lasers. Unfortunately, the requirements for micro-patterning architectures have grown more rapidly than technological advance in the trend of shortening wavelengths in such equipment. In recent years, it has been strictly required that micropatterning be done with the minimum device-feature length that shrinks to less than or equal to the wavelength of the exposure beam used. This minimum feature length shrinkage would result in a layout of IC components—particularly, memory cells—becoming more complicated in planar shape, which necessitates the use of irregular polygonal layout patterns including key-shaped components, in order to achieve the intended configuration of an on-chip circuitry with enhanced accuracy. This makes it impossible, or at least very difficult, to microfabricate ultrafine layout patterns while disadvantageously serving as the cause of the destruction of the symmetry of memory cells.
Regrettably, the prior art approach is associated with a need to curve or bend a diffusion layer into a complicated key-like shape for the purpose of making electrical contact with a substrate of the P-type well region. Thus, the prior art suffers from the problem of the degradation of the symmetrization of the cell layout pattern, making the successful achievement of microfabrication architectures for higher integration densities difficult.